- Growth Regulators
All classes of naturally occurring plant growth regulators (auxins, gibberellins, cytokinins, abscisic acid)
can be recovered in phloem saps, indicating that these compounds are normally translocated in the phloem
[30,39].
Cytokinins have been reported to regulate shoot development, possibly through regulation of sink ac-
tivity and changes in resource partitioning patterns. Zeatin and zeatin riboside have been reported to be
the dominant transportable forms of cytokinin in the plant [40,41]. Although the root is believed to be the
primary site of synthesis of cytokinins, Kamboj et al. [41] showed that zeatin riboside was the predomi-
nant form in the roots while zeatin predominated in the phloem sap. This indicated that zeatin riboside
was the predominant form translocated from roots via the xylem and that zeatin itself was the major form
transported via the phloem. The site of synthesis of zeatin in the phloem sap is still unknown. It has been
suggested that origin is through synthesis within the mature leaf followed by subsequent loading into the
phloem, through direct exchange with xylem cytokinin metabolites or through recirculation from the
roots.
- Systemic Signals
Grafting experiments using source leaves have indicated that other growth factors apart from the known
growth regulators are also translocated in the phloem. These include floral initiation signals, cold hardi-
ness–inducing signals, and pathogen resistance factors [30]. The chemical nature of these systemic sig-
nals is only beginning to be deciphered. Salicylic acid, which appears to be one prime candidate as a sig-
naling molecule for these responses in some plant species, is thought to be phloem mobile [42,43]. In
addition, a phloem-mobile peptide, systemin, has been shown to induce pathogen resistance [44]. The
plant growth regulator abscisic acid is a likely candidate as a phloem-mobile cold hardiness–inducing fac-
tor [45].
Phloem tissues may also be capable of limited synthesis of phloem-specific proteins, whose function
is unknown, but they may also be involved in signaling [46,47]. The localization of sucrose synthase
within phloem tissue [48,49] suggests that this enzyme may be involved in the signaling pathway that
leads to callose synthesis [48] in response to wounding or pathogen invasion. It is likely that many more
signaling mechanisms will be discovered in the phloem.
- Xenobiotics
A number of man-made chemicals of agronomic importance, including many herbicides and pesticides
[30], are also translocated in the phloem. Combinations of lipid permeability and acid dissociation con-
stants (pKa) are predictors of phloem mobility that have been validated for many compounds in various
plant systems. One particularly good systemic herbicide is glyphosate (N-phosphonomethylglycine),
which is highly mobile in the phloem (Table 3).
The limitations of phloem mobility appear to be due mostly to failure of the applied chemical to cross
cuticular barriers, retention along the phloem path (limitation of lateral efflux along translocation path)
PHLOEM TRANSPORT OF SOLUTES IN CROP PLANTS 457
TABLE 3 Symplastic (Phloem) Transported Herbicides
Herbicide class Typical representative Chemical structure
Phenoxy herbicides 2,4-D 2,4-Dichlorophenoxyacetic acid
2,4,5-T 2,4,5-Trichlorophenoxyacetic acid
Benzoic acids Dicambaa 3,6-Dichloro-2-methoxybenzoic acid
2,3,6-TBA 2,3,6-Trichlorobenzoic acid
Picolinic acids Piclorama 4-Amino-3,5,6-trichloropicolinic acid
Triclopyr [(3,5,6-Trichloro-2-pyridyl)oxy]acetic acid
Chlorinated aliphatics Dalapon 2,2-Dichloropropionic acid
Triazoles Amitrolea 3-Amino-s-triazole
Organic arsenicals DSMA Disodium methanoarsenate
Glyphosate N-Phosphonomethylglycine
Sulfonylureas Chlorsulfuron 2-Chloro-N-[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)
aminocarbonyl]benzenesulfonamide
aAlso transported apoplastically (xylem).